1. Field of the Invention
The present invention relates to flexible lead bonding machines for semiconductor devices. More particularly, this invention relates to bonding machines for simultaneously bonding a plurality of leads to semiconductor terminal pads. The flexible lead bonding machine of the type disclosed herein employes flexible printed circuits on sprocketed tape. Such bonding machines have been referred to as gang lead bonders, innerlead bonders, outerlead bonders, and flexible lead bonders.
The present invention is directed to a high force bonding system of the type capable of causing flexible conductive leads to exceed their yield point and to form a fusion molecular bond with the terminal pads on a semiconductor device. This invention is an improvement in our Flexible Lead Bonding Apparatus application Ser. No. 588,289 filed June 19, 1975.
2. Description of the Prior Art
The trend in the manufacture of semiconductor devices is to make larger and more complex integrated circuit devices. The number of active devices per unit of area base material is being increased which aids in making the individual active devices faster and cheaper. The internal connections between components and active devices in such semiconductors are made during the process of producing the integrated circuit wafers.
The single most time consuming assembly operation in the manufacture and packaging of semiconductor devices is the wire bonding or lead attaching operation wherein conductive wires or prefabricated leads are attached to the terminals of an integrated chip and the other end of each of the wires or prefabricated leads is connected to a substrate or supporting carrier assembly.
In the aforementioned application Ser. No. 588,289 there is described a bonding machine for making a plurality of bonds simultaneously and employing eutectic solder bonds. Eutectic solder is usually formed between tin plated flexible leads and gold terminal pads or between tin alloy solder terminal pads. Such eutectic solder bonds may be formed at forces well under a hundred grams per lead, assuming a sixteen square mil bonding area and temperatures around 300.degree. C.
Some integrated circuit manufacturers require the highest available reliability of their packaged semiconductor devices and generally conclude that gold terminal pads and gold covered conductive leads achieve this end. Bonding gold to gold requires that the compressive yield strength of the gold or gold alloy be exceeded. This is usually achieved by employing medium or high pressure to the thermode at high temperature to achieve thermocompression bonds. Such thermocompression bonds result in molecular fusion of the gold at the inner face of the bond to provide a continuous and homogenous connection. Such thermocompression bonds may be formed at forces around one hundred grams per lead, assuming a sixteen square mil bonding area and temperatures over 500.degree. C. A forty lead semiconductor device having 16 square mil terminal bonding pads would require a total force on the thermode of about 10 pounds. Terminal pads on larger devices may exceed one hundred square mils requiring about 70 pounds of pressure at the aforementioned high temperatures.
Some integrated circuit manufacturers require the cheapest reliable conductive lead connections to their semiconductor devices. Copper leads bonded to copper terminal pads or other equivalent conductive base nonferrous metals fill this need when the leads and pads are connected by clean molecular fusion bonds. To thermocompression bond copper to copper on a 16 square mil area requires approximately 1.63 pounds per lead. A 40 lead device would require about 26 pounds of bonding force on the thermode. When outerleads are being bonded, the area to be bonded approaches 100 square mils per lead and the thermode force necessary for bonding outerleads would be approximately 168 pounds.
When the conductive leads and terminal pads are made of high strength conductive nickel alloys, the forces necessary to make thermocompression bonds to a semiconductor device may approach 500 pounds.
Heretofore, high force thermocompression bonding systems have been made employing thermodes on the end of a hydraulic or pneumatic ram. Such devices have not been automated and are very slow. Further, such devices present problems in aligning the working face of the thermode with the conductive leads and the terminal pads to be bonded.
High pressure thermocompression bonding machines having automatic features have been made, however, such machines merely implement the mode of operation of the fluid ram mentioned above. These prior art devices mount one or more pneumatic cylinders on a moving carriage or slider and provide horizontal motion of the carriage, which supports the thermode. The thermode holder is mounted on vertical sliders on the horizontally movable carriage. After the thermode is positioned over the device to be bonded, the fluid pressure in the cylinder is increased slowly or in stages until the thermode clamps the leads between the device and the thermode. The high pressure fluid may then be applied. Retraction of the thermode to permit alignment of a new set of flexible leads with another semiconductor device is provided by reversing the motion of the cylinder, sliders, and carriage.
Prior art bonding machines were designed for a single narrow range of pressures. When prior art bonding machines were made for low pressure eutectic bonding they were not capable of making high pressure thermocompression bonds. Similarly, when prior art bonding machines were made for high pressure bonding they were not capable of providing accurate medium or low pressure forces at the thermode.